In telecommunications systems, electrical components in telecommunications nodes located at Distribution Points a short distance from the customer termination are powered from the customer end instead of, as was traditionally the case, using a “forward power feed” from the exchange end. This has become more common as connections have become more complex than a simple wire pair from exchange to customer, and now require powered components such as optical/electrical interfaces at intermediate points in the network. In such cases delivering the required power from the exchange end is at worst impossible, for example if the connection is made by optical fiber, and at best inefficient, because of losses in the electrical wiring. In particular, those parts of a connection towards the exchange can be made of relatively thin wire as the individual wire pairs are bundled with others, which gives the bundle greater mechanical integrity. Closer to the customer, the copper wiring is in smaller bundles, and eventually a single wire pair, so each individual wire has to be thicker than it is towards the exchange, in order to ensure its mechanical integrity. The drop lengths are also generally shorter. These factors result in significantly less ohmic loss being incurred if the customer provides the majority of the power. It is therefore more efficient to draw power from the customer end. Such an arrangement means that the system is being powered from the customer's electricity supply, and therefore paid for by the customer. However powering from the exchange end would be less efficient, and the extra cost of doing so would eventually have to be passed on to the customer.
However, relying on collecting power from the customer end involves some uncertainties in the delivery of the power supply, which complicates the management of such remote units.
The proposed ETSI TM6 standard for Reverse Power Feed (ETSI TR 102 629) will allow interoperability between the FttDP (Fiber to the Distribution Point) remote node and the various Customer Premises equipment (CPE) to be used to provide the power to this node. Such systems also provide a battery backup in the CPE to ensure continuity of service in the event of a failure of the external supply. However, if no CPE is connected, there is no power supply to the node. One of the characteristics of the remote node units is that they are often sited at remote locations with restricted access i.e. at the top of poles or in underground chambers. This makes it difficult to maintain a reliable source of battery power at the node. Consequently, when there are no customers (users) connected to a remote unit, the unit has no power supply.
Fiber to the Distribution Point (FttDP) has an inherent problem in that when there are no customers providing power (via Reverse Power Feed RPF) then the remote Drop Point Unit (DPU) becomes un-available to the element manager that is set to remotely control that unit. In response to this issue the Persistent Management Agent (PMA) concept has been developed which provides a proxy management capability for the remote unit and is always addressable even when the remote unit is unpowered. This is important as the element manager does not ‘know’ if the remote unit has become un-available due to an internal or infrastructure failure or if the remote unit simply has no customers connected providing electrical power. This has the consequence of alarms being raised when a DPU becomes un-available even though there is no equipment or infrastructure failure. Mechanisms such as ‘dying gasp’ may be used to indicate the nature of ‘how’ a DPU has become un-available. However, such mechanisms are not always reliable and can put other burdens upon the network management platform.
This raises problems for the network management system being used to control and monitor this remote element since it may not be apparent whether the unit is simply unpowered or has developed a fault, or indeed if the optical fiber providing the backhaul connection to the unit has been damaged. It is therefore desirable to have an additional power supply available to the unit. Providing a power connection to the distribution point from a local utility provider is often not a practical solution since it can be expensive, and the distribution points are often in publicly-accessible curbside locations where there can be public safety concerns in providing a mains electricity supply, as well as the possibility of unauthorized connection to the electricity supply.
From another perspective, RPF has to have a fair-sharing policy when multiple users are providing power since there are two separate areas that require power when describing the power consumption of a DPU:                1) The power required for the individual line modems (xDSL modem/Layer 2 switch).        2) The common electronics in the DPU such as fiber backhaul, power management, network management, supervision electronics and Layer 2 switching functions.        
When a sole user is connected to a DPU then that single user has to provide all the power. When further users connect, then each user provides the power for their individual modem and a share of power to feed the common electronic components.
High speed DSL systems such as VDSL2 and G.fast make use of crosstalk cancellation systems (also known as vectoring) which work by injecting an ‘anti’ crosstalk signal onto the near-end of other twisted pairs within a cable binder group such that the received signal at the far end of the cable is devoid of any crosstalk (for downstream vectoring). Obviously, as more active modems operate over a common binder group then the individual crosstalk signals that require cancellation become more complex. However, if crosstalk cancellation is being successfully operated between several users and one of the users then ‘hangs-up’, the power contributed by that user is lost. Until a new ‘vectoring matrix’ is calculated and switched over to replace the current matrix, the modem which is to leave the matrix has to continue to be powered. This means that until the system switches over to the new matrix, each modem remaining in the vector group has to operate with reduced performance in order to maintain power to all the modems, unless power can be obtained from somewhere else to keep the extra modem powered.
It is known to provide both forward and reverse power feeds to an intermediate node. In one example a backup facility takes power from the preferred (reverse) source with a failover system controlled by relays switching to a basic “lifeline” voice service, powered from the exchange end. Such a system is described in United Kingdom patent specification GB2319701 or International Patent specification WO 08/132428. However, this simple system requires a variable power draw from the customer end, depending on the services being used and the number of other users sharing the load of the common services at the node.
As noted above, forward power will generally be subject to greater resistive losses because of the longer lengths and generally thinner wiring available, and therefore to improve efficiency the remote unit should extract as much reverse power from the customer as possible (or allowed by the contract established between the customer and communications provider), to ensure that the cost of powering the remote node is mitigated as much as possible with regards to the communications provider.
If the node relies solely on power fed from the customer nodes, it has to determine how much power to draw from each one. This can be determined in part from the services being used, but may nevertheless result in unfairness, for example if one customer connection has a greater resistance than another (because it is further away, or because of damage to the line) it would need to put more power into the line to deliver the same amount at the node, to compensate for the greater ohmic losses in the line itself, despite the fact that those line properties are likely to result in a poorer service. It is known from European patent specification EP 2120443 for a distribution point to vary the power it draws from a customer line according to characteristics of the line itself, such as electrical resistance or the bitrate it can support. However, the management system in the node will not readily have information as to whether any of the individual customer nodes are subject to power supply problems, and may continue to draw power even when the customer is operating on battery power, until the battery is depleted and even the lifeline availability is lost.